­Soul Exchange

I don't recall this card at all. Let's see. Oh right, "Splodge of paint" art-style that would be why. Huh, sacc a creature (thrull) to reanimate something else; possibly making it significantly larger. So it's cretaure from graveyard to play; a black staple.

Also, revenge of the +2/+2 counter. So swap that out for a pair of +1s, we're left with a somewhat parasitic reanimation spell? Two mana does seem to be about the right place to put this. Double black instead of because of the potential +2? Could maybe be cheaper because you do have to find a chump to sacrifice to it. But sure, not every card has to be the best posisble thing it could be; it's not like the graveyard was a big theme in this set (that was the dark, except it kinda wasn't).

Having it common is a bold statement; so I think we just nudge this up to uncommon, fix up the wording a bit, and have done. Should I add "if you do?" to the sacrifice? You know what? Let's not. Now it has sneakily two potential upsides; either no creatures, or sacc a thrull are good things.

I think you intended the first word to be 'Sacrifice'.

Nope; the original exiles. Current oracle wording is: "As an additional cost to cast this spell, exile a creature you control." Original was "Sacrifice a creature, but remove it from the game instead of putting it in your graveyard" which was... clunky.

And exile gets around all the fiddly questions of "Can I sacrifice a thrull and bring it back with counters on it?"

Oh. Then I think you intended the second sentence to include the word 'exiled'.

So if you control no creature this is Zombify for two mana?

Thanks for the wording catch; didn't spot that. And as currently; yes it is. That might be too good, in which case I'd add back in the "If you do" words. But I think we'll try it this way.

Under modern wording, it would likely be an additional cost

The Oracle text of the original also is an additional cost. No need for "If you do" words either way.